Increasing the transmission data rate in a radio frequency (RF) communications system is typically desirable. The ability to simultaneously transmit multiple RF communications signals in multiple data channels in the same frequency band is one method of increasing the transmission data rate. FIG. 1 illustrates a transmit antenna array 102 and a receive antenna array 104 of a multiple input multiple output (MIMO) communications system 100. A MIMO system like system 100 utilizes the spatial separation of the antennas in the transmit array 102 and the receive array 104 to separate the data channels.
RF signals spread out as they propagate. Thus, multiple RF signals transmitted by the spatially separated antennas in the transmit array 102 will eventually overlap and interfere with each other sufficiently to make it difficult or impossible to separate the signals at the receive array 104. For a given intensity level of the transmitted RF signals from the transmit array 102, the maximum permissible distance between the transmit antenna array 102 and the receive antenna array 104 in which a MIMO communications systems 100 can resolve with relative certainty the transmitted RF signals is often termed the Rayleigh range R and is equal to the product of the distance across (e.g., the diameter of) DT the transmit antenna array 102 and the distance across (e.g., the diameter of) DR the receive array 104 divided by the wavelength λ of the center frequency of the frequency band in which the transmit array 102 antennas are transmitting. That is: R=(DT*DR)/λ. For example, all adjacent antennas in the transmit array 102 can be spaced a uniform distance apart in both the y and z directions in FIG. 1, where the distance D between the transmit array 102 and the receive array is in the x direction. As noted, the distance D should be less than or equal to the Rayleigh range R.
Although a MIMO communications system 100 can provide multiple channels for transmission of multiple RF signals in the same frequency band, the number of channels is limited to the number of antennas in the transmit array 102 for which there is a corresponding antenna in the receive array 104. Moreover, the distance D between the transmit array 102 must be less than the Rayleigh range R.
As is known, orbital angular momentum (OAM) can be imparted to RF signals, and RF signals in the same frequency band but with different OAM modes can be combined and transmitted as a composite OAM beam to a receiver, where the different OAM modes of the individual RF signals can be detected and the RF signals separated from the composite beam. FIG. 2 illustrates an example of a composite beam 200 comprising a first OAM signal 204 with a first OAM mode and a second OAM signal 214 with a second OAM mode.
As also known, an OAM signal 204, 214 turns (i.e., twists) about an axis 202 in the direction of propagation of the signal. For example, the wave front of the signal 204, 214 can be substantially spiral or helical. The numerical value of the OAM mode of such signals corresponds to the time or distance 206, 216 between one full revolution of the signal 204, 214 about the axis 202, and the sign of the OAM mode corresponds to the direction (e.g., right or left) of the revolutions of the signal 204, 214 about the axis 202.
Some embodiments of the present invention provide improvements in receiving and separating the OAM signals of a composite OAM beam.